It is intended for use in the Vim statusline - here's an example to enable it:

set statusline=[r%{HGRev()}]

HGRev exports the "HGRev()" function (for use in the statusline), and
a command called ":RefreshMercurialRev", which you can use to manually
update the revision information for the current buffer. If you've added
HGRev() to your statusline, you'll see the file revision, branch, tags,
and file status reflected -- if you used the statusline example above,
you should see something like this:

[r192+ default tip M]

The information displayed can be easily customized, and HGRev can also
automatically update the status line on file changes. (This behavior is
disabled by default -- see the documentation for details.)